The present invention relates to a metal foil provided with an electrically resistive layer having excellent adhesiveness, high peel strength, and a low variation in resistance value, and relates to a printed circuit board comprising the metal foil.
Copper foil is generally used as a wiring material of printed circuit boards. The copper foil is classified based on the production process into electrolytic copper foil and rolled copper foil. The thickness of copper foil can be arbitrarily controlled from very thin copper foil of 5 μm to thick copper foil of about 140 μm.
The copper foil is bonded to a substrate of a resin such as an epoxy resin or a polyimide resin and is used in a printed circuit board. The copper foil is required to guarantee sufficient strength of adhesiveness to a resin serving as the substrate. Accordingly, in many cases of using electrolytic copper foil, the rough surface, a so-called mat surface, generally formed during production of the foil is further roughened and is then used. Rolled copper foil is also used similarly after surface-roughening treatment in many cases.
Recently, it is proposed to further form a thin-film layer of an electrically resistive material on copper foil serving as a wiring material (see Patent Documents 1 and 2). An electronic circuit board must comprise an electrically resistive element, and the use of copper foil provided with a resistive layer allows formation of the electrically resistive element merely by exposing the electrically resistive layer provided to the copper foil with an etching solution such as a cupric chloride solution.
Consequently, the limited surface area of the board can be effectively used by embedding the resistance in the board, compared with conventional methods of surface-mounting chip resistive elements on boards by soldering.
In addition, restriction on design is reduced by forming the resistive element inside a multilayer board, and the circuit length can be shortened to allow improvements in electrical characteristics. Accordingly, the use of copper foil having a resistive layer makes soldering unnecessary or highly reduces soldering and allows a reduction in weight and an improvement in reliability. Thus, a board having an electrically resistive layer embedded therein has many advantages.
The copper foil used as a base for resistive components is subjected to a surface treatment for further forming a resistive layer thereon. The copper foil differs from one for general printed board wiring, but is the same in the point of securing the adhesive strength to a resin by roughening.
A resin is further bonded on the resistive layer. Accordingly, in evaluation of the adhesive strength of a resistive material, it is necessary to investigate both the strength between the copper foil and the resistive layer and the strength between the resistive layer and the resin. For example, in peel strength evaluation, peeling occurs from the interface with a weaker strength.
In any case, a larger surface roughness provides a higher adhesive strength. The adhesive strength is thought to be affected by surface roughness and other factors such as surface chemical species (elemental species).
Continual improvement of the performance is required in printed circuit boards. Simultaneously, due to formation of finer resistance circuits and a demand for improvement in high-frequency characteristics, a reduction in surface roughness of resistive components is required. In order to achieve such requirements, it is necessary to increase the adhesive strength without relying on surface roughness.
However, it has not been exactly comprehended whether the surface roughness of copper foil affects an electrically resistive layer; what degree of surface roughness can maintain the characteristics of an electrically resistive layer; and what means can be employed for maintaining peel strength between an electrically resistive layer and a resin even when the adhesive strength decreases by reducing the surface roughness.
Accordingly, in conventional metal foil having an electrically resistive layer, a vague process of increasing the roughness of the surface of a copper foil is employed for enhancing the peel strength, and the above-mentioned problems have not been solved.
The present invention is an improvement of the invention of Patent Document 3 filed before by the present applicant. The invention described in Patent Document 3 is all effective and can be applied to the present invention.    Patent Document 1: Japanese Patent No. 3311338    Patent Document 2: Japanese Patent No. 3452557    Patent Document 3: Japanese Patent Application No. 2007-295117